I think with someone on the spectrum difficulty in social anxiety might be a concern for wearing something that usually brings a lot of extra attention. Depending on whose around him he might get a bit of ridicule. That would be my #1 concern.

As far as the the kilt lifting and unwanted exposure, the clinch point on the kilt where you are most *aware* of it while you are wearing it is the high waist straps that hold it on. Lifting the kilt does 0 to alleviate that. However the inability to do most anything about that might frustrate your young man. UT Kilt has a "ultimate utility kilt" that has an elastic waist. I have one for working around the house and that mod does make the waist more comfortable and less confining. There is some personal rubbing the apron of a kilt can do when you are moving around. That would be a risk for kilt raising. Having a very soft to the touch liner would be a mitigation to any discomfort for that rubbing and any desire to lift the kilt to alleviate it.